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Register a new userThe H$alpha$ star formation main sequence in cluster and field galaxies at $zsim1.6$
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We calculate H$alpha$-based star formation rates and determine the star formation rate-stellar mass relation for members of three SpARCS clusters at $z sim 1.6$ and serendipitously identified field galaxies at similar redshifts to the clusters. We find similar star formation rates in cluster and field galaxies throughout our range of stellar masses. The results are comparable to those seen in other clusters at similar redshifts, and consistent with our previous photometric evidence for little quenching activity in clusters. One possible explanation for our results is that galaxies in our $z sim 1.6$ clusters have been accreted too recently to show signs of environmental quenching. It is also possible that the clusters are not yet dynamically mature enough to produce important environmental quenching effects shown to be important at low redshift, such as ram pressure stripping or harassment.
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We derive two-dimensional dust attenuation maps at $sim1~mathrm{kpc}$ resolution from the UV continuum for ten galaxies on the $zsim2$ Star-Forming Main Sequence (SFMS). Comparison with IR data shows that 9 out of 10 galaxies do not require further obscuration in addition to the UV-based correction, though our sample does not include the most heavily obscured, massive galaxies. The individual rest-frame $V$-band dust attenuation (A$_{rm V}$) radial profiles scatter around an average profile that gently decreases from $sim1.8$ mag in the center down to $sim0.6$ mag at $sim3-4$ half-mass radii. We use these maps to correct UV- and H$alpha$-based star-formation rates (SFRs), which agree with each other. At masses $<10^{11}~M_{rm sun}$, the dust-corrected specific SFR (sSFR) profiles are on average radially constant at a mass-doubling timescale of $sim300~mathrm{Myr}$, pointing at a synchronous growth of bulge and disk components. At masses $>10^{11}~M_{rm sun}$, the sSFR profiles are typically centrally-suppressed by a factor of $sim10$ relative to the galaxy outskirts. With total central obscuration disfavored, this indicates that at least a fraction of massive $zsim2$ SFMS galaxies have started their inside-out star-formation quenching that will move them to the quenched sequence. In combination with other observations, galaxies above and below the ridge of the SFMS relation have respectively centrally-enhanced and centrally-suppressed sSFRs relative to their outskirts, supporting a picture where bulges are built due to gas `compaction that leads to a high central SFR as galaxies move towards the upper envelope of SFMS.
We use high-resolution continuum images obtained at 870microns with the Atacama Large Millimeter Array (ALMA) to probe the surface density of star-formation in z~2 galaxies and study the different physical properties between galaxies within and above the star-formation main sequence of galaxies. This sample of eight star-forming galaxies at z~2 selected among the most massive Herschel galaxies in the GOODS-South field is supplemented with eleven galaxies from the public data of the 1.3 mm survey of the Hubble Ultra-Deep Field. ALMA reveals systematically dense concentrations of dusty star-formation close to the center of the stellar component of the galaxies. We identify two different starburst regimes: (i) the classical population of starbursts located above the SFR-M* main sequence, with enhanced gas fractions and short depletion times and (ii) a sub-population of galaxies located within the scatter of the main sequence that experience compact star formation with depletion timescales typical of starbursts of ~150 Myr. In both starburst populations, the far infrared and UV are distributed in distinct regions and dust-corrected star formation rates estimated using UV-optical-NIR data alone underestimate the total star formation rate. Starbursts hidden in the main sequence show instead the lowest gas fractions of our sample and could represent the last stage of star-formation before they become passive. Being Herschel-selected, these main sequence galaxies are located in the high-mass end of the main sequence, hence we do not know whether these starbursts hidden in the main sequence also exist below 10^11 Msun. Active galactic nuclei are found to be ubiquitous in these compact starbursts, suggesting that the triggering mechanism also feeds the central black hole or that the active nucleus triggers star formation.
We compare galaxy scaling relations as a function of environment at $zsim2$ with our ZFIRE survey where we have measured H$alpha$ fluxes for 90 star-forming galaxies selected from a mass-limited [$log(M_{star}/M_{odot})>9$] sample based on ZFOURGE. The cluster galaxies (37) are part of a confirmed system at z=2.095 and the field galaxies (53) are at $1.9<z<2.4$; all are in the COSMOS legacy field. There is no statistical difference between H$alpha$-emitting cluster and field populations when comparing their star formation rate (SFR), stellar mass ($M_{star}$), galaxy size ($r_{eff}$), SFR surface density [$Sigma$(H$alpha_{star}$)], and stellar age distributions. The only difference is that at fixed stellar mass, the H$alpha$-emitting cluster galaxies are $log(r_{eff})sim0.1$ larger than in the field. Approximately 19% of the H$alpha$-emitters in the cluster and 26% in the field are IR-luminous ($L_{IR}>2times10^{11} L_{odot}$). Because the LIRGs in our combined sample are $sim5$ times more massive than the low-IR galaxies, their radii are $sim70$% larger. To track stellar growth, we separate galaxies into those that lie above, on, and below the H$alpha$ star-forming main sequence (SFMS) using $Delta$SFR$(M_{star})=pm0.2$ dex. Galaxies above the SFMS (starbursts) tend to have higher H$alpha$ SFR surface densities and younger light-weighted stellar ages compared to galaxies below the SFMS. Our results indicate that starbursts (+SFMS) in the cluster and field at $zsim2$ are growing their stellar cores. Lastly, we compare to the (SFR-$M_{star}$) relation from RHAPSODY cluster simulations and find the predicted slope is nominally consistent with the observations. However, the predicted cluster SFRs tend to be too low by a factor of $sim2$ which seems to be a common problem for simulations across environment.
The redshift range z=4-6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understand early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star-formation history (SFH) of 221 $zsim4.5$ main-sequence galaxies at log(M) > 9.7 by comparing their ultra-violet (UV) continuum, H$alpha$ luminosity, and H$alpha$ equivalent-width (EW). The H$alpha$ properties are derived from the Spitzer [3.6$mu$m]-[4.5$mu$m] broad-band color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between H$alpha$ and UV-derived luminosities and star-formation rates (SFRs). About half of the galaxies show a significant excess in H$alpha$ compared to expectations from a constant smooth SFH. We also find a tentative anti-correlation between H$alpha$ EW and stellar mass, ranging from 1000$r{A}$ at log(M) < 10 to below 100$r{A}$ at log(M) > 11. Consulting models suggests that most $zsim4.5$ galaxies had a burst of star-formation within the last 50 Myrs, increasing their SFRs by a factor of > 5. The most massive galaxies on the other hand might decrease their SFRs, and may be transitioning to a quiescent stage by z=4. We identify differential dust attenuation (f) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing H$alpha$ EW (reaching values similar to high-z galaxies), we predict that f approaches unity at $z>4$ consistent with the extrapolation of measurements out to z=2.
To investigate the variability of the star formation rate (SFR) of galaxies, we define a star formation change parameter, SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ which is the ratio of the SFR averaged within the last 5 Myr to the SFR averaged within the last 800 Myr. We show that this parameter can be determined from a combination of H$alpha$ emission and H$delta$ absorption, plus the 4000 A break, with an uncertainty of $sim$0.07 dex for star-forming galaxies. We then apply this estimator to MaNGA galaxies, both globally within Re and within radial annuli. We find that galaxies with higher global SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ appear to have higher SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ at all galactic radii, i.e. that galaxies with a recent temporal enhancement in overall SFR have enhanced star formation at all galactic radii. The dispersion of the SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ at a given relative galactic radius and a given stellar mass decreases with the (indirectly inferred) gas depletion time: locations with short gas depletion time appear to undergo bigger variations in their star-formation rates on Gyr or less timescales. In Wang et al. (2019) we showed that the dispersion in star-formation rate surface densities $Sigma_{rm SFR}$ in the galaxy population appears to be inversely correlated with the inferred gas depletion timescale and interpreted this in terms of the dynamical response of a gas-regulator system to changes in the gas inflow rate. In this paper, we can now prove directly with SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ that these effects are indeed due to genuine temporal variations in the SFR of individual galaxies on timescales between $10^7$ and $10^9$ years rather than possibly reflecting intrinsic, non-temporal, differences between different galaxies.
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